1. Field of the Invention
This invention relates to a recording head for forming flying small droplets by discharging a recording liquid generally called ink from a discharge port, and more particularly to an ink jet recording head where water repellency is improved for a peripheral portion of the discharge port.
2. Related Background Art
FIG. 1 is a perspective development illustrating a structure of a conventional ink jet recording head. This recording head is provided with a discharging pressure generating element 12 located on a substrate 11 made of glass, ceramics or the like. By patterning a photo-sensitive resin cured film 13 by a photolithography method, there are formed a liquid pathway 15 corresponding to the discharging pressure generating element 12, a discharge port 14, and a liquid chamber 16. On the photo-sensitive resin cured film 13, there is secured by a bonding agent 19 a top plate 17 made of, for example, glass, ceramics, metal or the like. The top plate 17 is formed with a recording liquid supply port 18.
In the recording head constructed as described above, the physical properties of the substrate 11 around the discharge port 14, the photo-sensitive resin cured film 13, and the surface of the top plate 17 are extremely important for constant and stable discharge of the recording liquid from the discharge port 14. More specifically, if the recording liquid invades around the periphery of the discharge port 14 to cause a liquid pool in part thereof, this liquid pool affects the recording liquid in the pathway 15 such that the flying direction of the recording liquid, when discharged from the discharge port, deviates from a predetermined normal direction. Further, due to an unstable state of the liquid pool, the flying direction of liquid droplets is disturbed every time they are discharged, thereby hindering stable liquid discharge and accordingly satisfactory recording.
Furthermore, a so-called splash phenomenon, i.e., the entire periphery of the discharge port 14 is covered with a recording liquid film may arise to cause scattering of the recording liquid, which leads to hindering stable recording. Also, when a liquid pool covering a peripheral portion of the port becomes larger, the recording head even falls into a liquid discharge disabled condition.
The members surrounding the discharge port of the recording head as shown in FIG. 1 are in many cases made of different materials from each other. For example, the substrate 11 is made of silicon; the top plate 17 is made of glass; and the photo-sensitive resin cured film 13 is made of resin. The recording liquid is most likely to leak from a portion made of the most susceptible material to leakage of the three kinds of materials used in the peripheral portion of the discharge port. Among these three kinds of materials, glass is most susceptible to leakage of normal ink, so that ink possibly leaks from a portion made of glass. However, the glass is generally used because of its favorable properties suitable to the production of heads, so that substitution of another material for glass for the purpose of preventing leakage of ink is not desirable in view of production, performance and cost-effect of the recording head.
To solve the problem that a liquid pool of a recording liquid in a peripheral portion of a discharge port hinders the recording liquid from being stably discharged, as described above, conventional recording heads have the peripheral portion of the discharge port 14 subjected to a so-called liquid repellency treatment to form a liquid repellency treated layer 20 which sheds ink. A number of proposals for solving the above-mentioned problem by this water repellency treatment have already been open to the public. As agents used for this liquid repellency treatment, there are a variety of materials, for example, silicon-group polymer and oligomer, fluorine-group polymer and oligomer, and so on.
The liquid repellency treated layer 20 formed on an ink jet recording head must have not only favorable water repellency but also sufficient durability. The durability of such liquid repellency treated layer will hereinafter be discussed.
For implementing an ink jet recording method, even with a liquid repellency treated peripheral portion of a discharge port, the peripheral portion of the discharge port is always in contact with a recording liquid, so that a recovery operation is generally performed such that the discharge port surface is wiped by an absorbing member made of polyurethane foam or the like to remove ink attached thereon. Therefore, the water repellency treated layer is required to have adhesive property and wear resistance to an extent that it is not peeled or destroyed, even if wiped by an absorbing member. If these properties are not sufficient, even if initial performance and effects may be satisfactory, the water repellency treated layer will be gradually peeled, destroyed, or present no water repellency effects while a recording head is being used for a long term, which leads to hindering a stable discharging operation for printing. There have been found several cases where a water repellency treated layer formed by a conventional water repellency treating agent is not sufficient for the durability required to such an ink jet recording head. Specifically, referring to FIGS. 3A-3C, consider a case where a rubber blade is used to remove ink and foreign substances attached on a dischare port surface. When a relatively soft fluorine compound of a conventional type is used as a water repellency treating agent, fragments of a layer made of the water repellency treating agent, scraped off by the rubber blade or the like (FIG. 3A), invade even into the discharge port of the recording head (FIG. 3B). If the scraped water repellency treating agent fragments invade into the discharge port, the meniscus position of ink is offset (FIG. 3C), thereby giving rise to slippage and consequently deteriorating the quality of printed characters. For this reason, there has been a need to a water repellency treated film which is highly resistant to wiping by a rubber blade or the like and has a high hardness.
With an ink jet recording head which has members around a discharge port made of a plurality of different materials, it is necessary to form a liquid repellency treated layer which exhibits a good adhesive property with all materials. Liquid repellency treated layers formed by conventional liquid repellency treating agents are sometimes insufficient particularly in this respect. On the other hand, with an ink jet recording head employing a groved top plate having a liquid chamber, a liquid pathway and a discharge port surface integrally formed therein, the grooved top plate is made of a single material since it is molded. Even if a top plate is made of a single material, such a material must be selected in many cases from a limited number of materials due to restraints such as the properties thereof associated with easiness in molding, ink contact property and so on. While materials such as polysulfone, polyethersulfone and so on are generally employed, many of these materials do not have a functional group reactive with other materials, thereby the adhesive property with the liquid repellency treating agent is not sufficient, thereby incurring problems of peel-off of the liquid repellency treated layer.
Even if wear resistance is ensured by using a high polymer liquid repellency treated layer with a relatively high hardness sufficient adhesive property is not provided, which may occasionally result in the liquid repellency treated layer peeled from a peripheral portion of a discharge port as shown in FIGS. 4A, 4B. Furthermore, when the liquid repellency treatment is performed with a high polymer liquid repellent agent with a high hardness after forming a discharge port, liquid repellency treated layer is sometimes formed in a manner that it covers the discharge port due to its good film forming property. Even if a so-called silane coupling agent is mixed with a high polymer liquid repellent agent to compensate for lack of adhesive property, the liquid repellency is degraded when it is in use.